1. Field of the Invention
This invention relates to a signal level adjusting unit for adjusting a level of an eight-fourteen-modulation (EFM) signal to a given level, which EFM signal is read from an optical disk in an optical disk player such as a compact disk (CD) player, laser disk (LD) player or the like.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows a circuit, which is used to reproduce EFM signals indicative of read digital data of an optical disk in a conventional 3-beam type optical disk player. Such an optical disk player uses three beams, i.e. a main beam for digital data reading and focus control and two sub-beams for tracking control. Referring to FIG. 1, the main beam reflected on the optical disk is detected by four photodiodes 1-4. A signal detected by the photodiodes 1 and 2, and a signal detected by the photodiodes 3 and 4 are amplified by amplifiers 5 and 6, respectively, and are added by an adder 7. Thus, the EFM signals are reproduced, amplified by an amplifier 8, and transmitted to succeeding circuits via an output terminal 9.
Thereafter, the EFM signals are subject to clock signal processing and error correction processing, and then are demodulated into audio data. The EFM signals may have a waveform as shown in FIG. 2A if laser beams pass on a compact disk which is defective on its surface. Further, the EFM signals may have a waveform as shown in FIG. 2B if the laser beams pass across a plurality of rows of tracks. The EFM signals not only include data such as audio data but are also used to detect a defect on the optical disk on the basis of the waveform as shown in FIG. 2A or determine a position of the main beam on the optical disk during access on the basis of the waveform shown in FIG. 2B.
Therefore, the EFM signals are very important from the viewpoint of reproducing sounds and performing subsequent processing and detection. Use of the circuit of FIG. 1 allows the reproduction of the EFM signals.
In the 3-beam type optical disk player, when detecting reflected beams by letting the main beam track a pit row, an amount of light in the reflected beams varies with a size of the pit. Therefore, peak and bottom values of an EFM signal are variable as shown in FIG. 2C. Further, optical disks have different values of reflectance, and reflect different beams of differing extents. Thus, signal levels of the EFM signals are different in optical disks. Further, the peak and bottom values of the EFM signals are different in respective optical disks.
The EFM signals undergo waveform shaping so as to facilitate processing in succeeding stages. According to the circuit shown in FIG. 1, the EFM signals are only amplified by the amplifier 8. If amplifiers 5 and 6 have small gains, the levels of the EFM signals will be low, so that the EFM signals cannot be correctly waveform-shaped due to noise. To overcome this problem, the gains of the amplifiers 5 and 6 are increased so that the EFM signals have a high level. Then, the EFM signals are waveform-shaped. However, if detected EFM signals have a high level, they may be clipped when they are amplified by the dynamic ranges of the amplifiers 5 and 6.
There is another problem as described below. Variations of the level b (i.e. the bottom value) of the EFM signals are detected on the basis of the waveform as shown in FIG. 2(B) so as to detect whether the laser beams pass across a plurality of rows of tracks. In such a case, since optical disks have different EFM signals, sometimes the detected level b may be below a predetermined threshold level. Therefore, the variations of the level b cannot be detected, and it is not possible to determine whether the laser beams pass across a plurality of rows of tracks.